This is step towards separating the GCN and R600 tablegen'd code.
This is a little awkward for now, because the R600 functions won't have the
MCSubtargetInfo parameter, so we need to have AMDMGPUInstPrinter
delegate to R600InstPrinter, but once the tablegen'd code is split,
we will be able to drop the delegation and use R600InstPrinter directly.
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